1. Field of the Invention
This invention relates generally to apparatus for converting solar energy to electricity by means of solar cells and more particularly to apparatus of this type which more efficiently collects and concentrates available sunlight for utilization by such cells.
2. Description of the Prior Art
It is well known that specially prepared semi-conductor p-n junctions known as photovoltaic cells can convert energy from sunlight to electricity. It is also known that such cells can convert to electricity only that portion of the incident photon energy spectrum, typically solar radiation, which creates hole-electron pairs within a given semi-conductor material. Photons having less energy than the desired spectrum are not absorbed at all, while more energetic photons are strongly absorbed and wasted in heating the cell which further degrades its energy conversion efficiency. Thus, it is advantageous to convert as much of the available light as possible into an energy spectrum to which a photovoltaic cell can usefully respond before the light strikes the cell's surface.
An existing technique for achieving the energy conversion alluded to above takes advantage of the fact that light falling on a luminescent material or agent is characteristically reradiated or emitted in such a fashion as to approach a narrow band of wavelengths of known energy content. Also light absorbed by such a material in one direction is scattered in many other directions. Materials or agents for this purpose include, for example, organic dyes used in scintillation counters, lasers, and the like. For the purpose of this application, the term "luminescent agent" is understood to include materials exhibiting all species of luminescence, including but not limited to fluorescence and phosphorescence.
It is shown in the literature that a dispersal of such luminescent materials within an internally reflective sheet or layer of transparent glass or plastic, one of whose major sides or surfaces is exposed to the sun, concentrates and focuses a flux of light of known energy level, by successive reflections, toward one or more of the upstanding edge faces of such sheet. If a photovoltaic cell responsive only to light as that known energy level is placed against or optically coupled to one such edge face, the energy conversion efficiency of the cell is increased several fold. In the present application, a light transmissive sheet of such construction and properties is termed a "luminescent sheet" and a photovolatic solar collector employing such a sheet may be termed a "luminescent solar collector". A luminescent solar collector of this type is fully and completely disclosed in Applied Optics, Volume 15, No. 10, Pages 2299-2300, dated October 1976, the disclosure of which is incorporated by reference.
While a thin luminescent sheet of the type described produces a high multiplication of incident light intensity on an edge mounted solar cell, such an arrangement is impractical for solar cells of conventional design and size.